In recent years, various attempts have been made to stimulate bone growth. These approaches have been essentially ad hoc, with no consistent framework within which to identify the most effective stimulation.
Kaufman et al., U.S. Pat No. 5,309,808 discloses an apparatus and method for therapeutically treating and/or quantitatively evaluating bone tissue In vivo. The Kaufman method includes subjecting bone to an ultrasonic acoustic signal pulse of finite duration, and involving a composite sine-wave signal consisting of plural discrete frequencies. These frequencies are spaced in the ultrasonic region to approximately 2 MHz; the excitation signal is repeated substantially in the range from 1 to 1000 Hz.
Duarte, U.S. Pat. No. 4,530,360 discloses an apparatus and a method of using ultrasonic energy for therapeutic treatment of bone tissue in vivo, using a pulsed sine wave at substantially a single frequency within the range from 1.3 to 2.0 MHz, and at a pulse repetition rate of 100 to 1000 Hz.
McLeod et al., U.S. Pat. Nos. 5,103,806 and 5,191,880 disclose methods for promoting bone tissue growth and the prevention of osteopenia, using mechanical loading of the bone tissue. In both patents, the inventors apply a mechanical load to the bone tissue at a relatively low level on the order of between about 10 and about 1000 microstrain, peak to peak, and at a frequency in the range of about 10 to 100 Hertz.
Bassett et al., U.S. Pat. No. 4,928,959 disclose a method and device for providing active exercise treatment for a patient suffering from a bone disorder. A patient is subjected to an impact load in order to stimulate bone growth, with an impact load sensor being used to monitor the treatment strength. The inventors noted that high frequency (up to 100,000 Hz) force components were important for stimulating bone growth.
Numerous other patents disclose methods for stimulating bone growth relying on the generation of electromagnetic signals. For example, Ryaby et al., U.S. Pat. Nos. 4,105,017 and 4,315,503 describe methods for promoting bone healing in delayed and nonunion bone fractures, using an asymmetric pulsed waveform. In U.S. Pat. No. 4,993,413, McLeod et al. disclose a method and apparatus for inducing a current and voltage in living tissue to prevent osteoporosis and to enhance new bone formation. They disclose the use of a symmetrical low frequency and low intensity electromagnetic signal substantially in the range of 1-1000 Hertz. In Liboff et al., U.S. Pat. No. 5,318,551 (and others), methods are disclosed that incorporate the combined use of a static and time-varying magnetic field to stimulate bone healing and growth. Specific amplitudes and frequencies are disclosed for optimal enhancement of bone growth, based on the theory of "ion-cyclotron resonance".
A recent publication by Weinbaum et al. provides a comprehensive and theoretically consistent basis to characterize the means by which bone growth occurs. The seminal article "A model for excitation of osteocytes by mechanical loading-induced bone fluid shear stresses," may be found in the Journal of Biomechanics, Vol. 27, 1994, pp 339-360. In this publication, they propose that it is not strain magnitude but strain rate that is the primary relevant variable responsible for adaptive bone growth and remodeling.
The prior art, exemplified by the references that have been briefly discussed, have used primarily ad hoc approaches or empirical findings for determining exogenous stimulations used up to now for the promotion of bone growth and healing. Some have focussed on the generation of specific values of biomechanical strain in the tissue as the primary modality of action. However, this invention incorporates the realization that fluid flow induced in normal physiological loading is the critically important variable in bone healing, and moreover, includes an efficient means for generating this fluid flow in living tissues. Specifically, this invention includes a means for stimulating fluid flow at relatively high frequencies by taking unique advantage of the nonlinear characteristics of ultrasound propagation in an ionic-fluid-saturated porous medium such as bone. In addition, this invention incorporates the important features of repetitive stimulation in analogy to that found in normal physiologic loading, and of adaptive feedback control for ensuring that an optimal signal dose arrives at the desired bone tissue site.